Fix ECS resource detector bug

[bhautikpip]

1. Fixes nil pointer bad reference panic when initialize resource detector as below.

ecsResourceDetector := new(ecs.ResourceDetector)

probably now with the fix we can initialize resource detector as below:

ecsResourceDetector := ecs.NewResourceDetector()

2. Minor changes
   Reference Issue: ECS resource detector crash aws-observability/aws-otel-go#16

[codecov]

Codecov Report

Merging #569 (c9c1bca) into main (b87d221) will decrease coverage by 0.0%.
The diff coverage is 0.0%.

@@           Coverage Diff           @@
##            main    #569     +/-   ##
=======================================
- Coverage   77.9%   77.9%   -0.1%     
=======================================
  Files         55      55             
  Lines       2593    2594      +1     
=======================================
  Hits        2022    2022             
- Misses       441     442      +1     
  Partials     130     130             

Impacted Files	Coverage Δ
detectors/aws/ecs/ecs.go	53.8% <0.0%> (-2.2%)	⬇️

[bonclay7]

Following the issue I've opened here, I went ahead and did a test with this implementation and it working very great for me on ECS with the following modifications

replace go.opentelemetry.io/contrib/detectors/aws/ecs => github.com/bhautikpip/opentelemetry-go-contrib/detectors/aws/ecs v0.16.1-0.20210207012249-86c04d1b71f1

ecsResourceDetector := ecs.ResourceDetector{Utils: new(ecs.DetectorUtils)}
ecsResource, err := ecsResourceDetector.Detect(ctx)

[bhautikpip]

@MrAlias addressed your comments!

[XSAM]

LGTM